The present invention relates to pulverulent materials, to mixtures thereof, and to the use of surface- and structure-modified pyrogenically prepared metalloid or metallic oxides as flow regulators.
Pulverulent materials or their mixtures have a tendency to caking during prolonged storage.
It is known to add flow improvers or anticaking agents to pulverulent materials.
Known flow improvers are pyrogenic or precipitated, hydrophobic or hydrophilic silicas, silicates or metallic oxides. Pyrogenic silicas have higher purity compared with products prepared by precipitation. Particularly effective flow regulators (=synonymous with flow improvers or anticaking agents) are hydrophobic pyrogenic silicas, which are prepared by after-treatment of pyrogenic silicas with, for example, alkylsilanes or silicone oils. Known examples are AEROSIL R972, AEROSIL R974, AEROSIL R812 and AEROSIL R202 (manufactured by Degussa AG).
Those known flow improvers and anticaking agents have the disadvantage that they do not always adequately improve the flowability. In addition, they are difficult to incorporate because they have a tendency to form agglomerates.
If, for example, it is desired to use pyrogenic silicas as flow regulators, it is necessary to ensure that the agglomerates are comminuted mechanically to such an extent that the silica can develop its full effectiveness. This can be achieved by the following measures, for example:                sieving the silica or the metallic oxide before it is mixed with one or more pulverulent materials,        preparing a pre-mix of the pulverulent material and the silica or metallic oxide, which is then sieved,        long mixing times and/or        the use of mixing devices which exert sufficiently high shear energy to adequately comminute the agglomerates of the silica or of the metallic oxide.That known process for the preparation of mixtures of pulverulent materials containing pyrogenic silicas or metallic oxides as flow regulators is accordingly relatively intensive in terms of time and energy. In addition, suitable devices, such as, for example, sieving machines or mixers, must be present.        